The in-trail procedure (ITP) is a protocol followed by an aircraft that desires to change its current flight level to a new flight level by descending or climbing in front of or behind one or more potentially blocking aircraft that are flying at an intervening flight level. In accordance with the ITP, certain conditions must be satisfied before the flight crew member issues a request for clearance to proceed with the flight level change. Whether or not the conditions are satisfied will depend on a number of dynamically changing factors associated with the host aircraft and other aircraft, such as the current geographic position of the aircraft, the current speed of the aircraft, the current heading of the aircraft, the desired new flight level, and the current flight level.
Modern flight deck instrumentation might include a vertical situation display (VSD) that provides a two-dimensional representation of an aircraft, the aircraft vertical flight plan, and neighboring aircraft. VSDs typically include a number of parameters and visual indicators that enable the pilot to form a quick mental picture of the vertical situation of the host aircraft. For example, VSDs may include displays of an aircraft symbol, the aircraft altitude, the vertical flight plan, and terrain. Thus, a member of the aircraft flight crew can obtain information related to the vertical situation of the aircraft relative to the terrain with a simple glance at the VSD.
A VSD could be used to identify the vertical position of potentially blocking aircraft for purposes of the ITP. Even if a VSD is deployed, however, the flight crew member will still need to mentally interpret the traffic situation and/or perform manual calculations related to the designation of potentially blocking aircraft and related to the determination of whether the conditions exist requiring the ITP protocol be used for a desired flight level change.